1. Field of the Invention
The present invention relates to a multi-beam scanner, a multi-beam scanning method, a method for detecting a synchronizing beam in multi-beam scanning, and an image forming apparatus.
2. Discussion of the Background
There has recently appeared a multi-beam scanning method for light scanning that can meet requirements for increasing speed in light scanning.
The multi-beam scanning method employs a plurality of light sources which can be independently modulated according to an image signal. Typically, one of those light sources may be a semiconductor laser where the emitted light wavelength varies slightly over different production lots of the laser.
If multi-beam scanning is conducted by combining a plurality of semiconductor lasers having different emitted light wavelengths, the chromatic aberration of a scanning optical system positioned between the light source and the scanned surface has the effect that the scanning optical system gives different optical actions for different beams (having different wavelengths). Therefore, if a long line is written in a sub-scanning direction, a phenomenon called vertical line fluctuation occurs where the thus written line wavers minutely.
The vertical line fluctuation typically appears notably on the write-in termination side of light scanning. Even if an image other than a vertical line is written to a portion with a notable vertical line fluctuation, an image density difference due to the shift in superposing of dots written in by the plurality of beams appears.
One of the methods for avoiding such a phenomenon is to use an achromatism-processed optical system as the above-mentioned scanning optical system. An achromatism-processed scanning optical system, however, is expensive in manufacturing costs and so contributes to increased costs of the relevant multi-beam scanner as a whole.
Another method may be a method for making up a light source apparatus by combining semiconductor lasers having an emitted light wavelength difference not larger than a predetermined value, as disclosed in Japanese Patent Application Laid-Open No. Hei 9-76562, by which, however, the vertical line fluctuation which is caused by the difference in the emitted light wavelengths of the combined beams cannot be mitigated further.